Due to recent changes in the species, size and quality of standing timber available for harvest throughout the world, composites of lignocellulosic materials have replaced traditional solid sawn lumber for use in many structural applications. Many of these composites are used in applications which require resistance to wood-destroying organisms such as fungi and various insects. Accordingly, this requires treatment with a wood preservative.
Traditionally, solid wood products are dipped or pressure treated with solutions of preservative chemicals. However, the nature of a composite material makes it possible to incorporate a preservative into the product during its manufacture. This decreases total production costs and yields a superior product in which the composite has a constant loading of preservative throughout its thickness.
Borates have been used as broad-spectrum wood preservatives for over 50 years. Their benefits include efficacy against most wood destroying organisms such as fungi, termites and wood-boring beetles. Coupled with their low acute mammalian toxicity and low environmental impact, their fungicidal and insecticidal properties have resulted in them being considered the wood preservative of choice for most structural or construction applications. Borates such as boric acid, borax, disodium octaborate tetrahydrate (sold as TIM-BOR® wood preservative, a product of U.S. Borax Inc.) and, more recently, zinc borate are well accepted as wood preservatives. Generally, boric acid, borax and disodium octaborate are used for treating solid, wood products by dip or pressure treatment. However, these preservatives are readily soluble in water and can be incompatible with many resin systems used in producing composite products, resulting in an adverse effect on the internal bond strength of the resultant composites and poor mechanical strength. Anhydrous borax and zinc borate have been used successfully at relatively low levels with some resin systems, such as the phenol-formaldehyde resins, to produce composites with acceptable internal bond strength. See Knudson et al., U.S. Pat. No. 4,879,083. Although the low solubility borates of Knudson et al, especially zinc borate, have been used successfully to treat wood composites such as oriented strand board (OSB), fiberboard, waferboard and particleboard, they suffer from several problems in actual commercial use. For example, in working with composites containing zinc borate, metal tools, such as saws, grinders and similar cutting tools may suffer significant wear and premature failure due to the borate's hardness. Also, the disposal of treated wood products by combustion can lead to problems in operating performance and maintenance of furnaces. It has also been found that particulate zinc borate used to treat wood composites has poor bulk flow properties which can cause difficulties in the wood composite manufacturing process.
The increased demand for treated wood composite products has resulted in a large volume utilization of borates in high capacity wood composite manufacture. Due to the very high volume throughput of commercial wood composite manufacturing facilities combined with the practice that waste wood is utilized as an energy source for wood particle drying as part of the process, an excessive build up of glassy borate deposits can occur within the furnaces. This will reduce the operating performance of the furnace as well as corrode the refractories of the furnace. In addition, the glassy borate deposits can be very difficult to remove from the furnace. See Daniels and Krapas, “Combustion Characteristics of Zinc Borate-Impregnated OSB Wood Waste in an Atmospheric Fluidized Bed,” 32nd International Particleboard/Composite Materials Symposium Proceedings, Mar. 31–Apr. 2, 1998, page 167 (1998).
Another type of lignocellulosic-based composite which can benefit from this invention are woodfiber-plastic composites. These composites, which are derived from wood and thermoplastic resin, are typically used in exterior applications such as decks and walkways. When used in exterior applications these products are subject to attack by mold and decay fungi. See Morris et al., “Recycled plastic/wood composite lumber attacked by fungi,” Composites and Manufactured Products, January 1998, pages 86–88; Mankowski et al., “Patterns of fungal attack in wood-plastic composites following exposure in a soil block test,” Wood and Fiber Science, 32(3), 2000, pp. 340–345; and Verhey et al., “Laboratory decay resistance of woodfiber/thermoplastic composites,” Composites and Manufactured Products, September 2001, pages 44–49. Unlike solid wood, these woodfiber-plastic composite products cannot be pressure-treated with preservatives and it is only possible to introduce the preservative treatment during the manufacture of the composite
This invention provides composites made from wood and other lignocellulosic materials which are resistant to attack by wood destroying organisms such as fungi and insects, have excellent internal bonding strength and may readily be cut, sawn and machined without excessive wear to the tools. Further, trimmings and other waste from manufacture and use of the treated composites may be disposed of by combustion without significant problems such as clogging and deterioration of the furnaces.